Hi, founder of https://tardis.dev here. Happy to answer any questions you have.
Have you built an API based business before?
No, it's my first one, perhaps it's obvious by looking at some aspects of the API and site, but did my best I could given my 'limitations'. Would love to hear some feedback on it.
“Per data day”. Is this pricing per single date?
Yes, pricing is per single date, initial version was 'per month' but that was too restrictive IMHO. With per day pricing one can order exact historical data that is necessary for the task and nothing more also mental calculation from day to month/year is rather quick.